Use NULL values in species table

[peterdesmet]

@cmspinto I notice the Species list API returns a mixture of null and "" values for some fields. Could this be updated in the database so empty values are consistently returned as null?

• esas_code: no empty values
• scientific_name: consistently null
• english_name: no empty values
• euring_code: mostly null, but 2 "" values
• euring_scientific_name: mostly null, but 2 "" values
• aphia_id: no empty values
• aphia_scientific_name: no empty values
• remarks: null, mostly "" and 8 null values

Let me know if this should pass by the DIG repo first